This invention relates to current sources and, more particularly, to a current mirror circuit arrangement for providing an output current that is proportional to an input current supplied to the current mirror circuit and which the ratio of the two currents is both trimmable and temperature independent.
Monolithic integrated current mirror circuits are well known to those skilled in the art. A simple current mirror circuit that is known includes an output transistor whose collector provides the output current and whose emitter is returned to a reference potential and a diode whose anode-cathode is connected in parallel with the base-emitter junction of the transistor. The input current is supplied to the anode of the diode. The diode is typically formed by an additional transistor which has its collector interconnected to both its base and the base of the output transistor. The ratio of the output current I.sub.O to the input current I.sub.T can be set to a desired value by area ratioing the emitters of the two transistors as is understood.
It is sometimes desirable to trim the two currents to a desired ratio while maintaining the trimmed ratio independent of temperature variations. One method to trim the current ratio in the above described current mirror circuit is to trim the emitter areas of the two transistors with respect to one another. Although emitter area trimming results in a ratio that is independent of temperature, it is not a practical method to be used.
Another method for adjusting the current ratio of the current mirror circuit is to provide resistive trimming using trimmable resistors coupled respectively between ground reference and the diode as well as the emitter of the output transistor. By trimming one or the other or both resistors, the value of I.sub.O can be adjusted with respect to a given input current I.sub.T. However, the current density of the output transistor is changed relative to that of the diode-connected transistor as one or the other resistor is trimmed. This produces a current ratio having some temperature coefficient (TC) other than zero which may not be desirable.
Hence, a need exists for a method for providing trimmable current ratios in such current mirror circuits wherein the adjusted ratio is independent to temperature variations, i.e., a zero TC.